Plasma chamber for controlling ion dosage in ion implantation

ABSTRACT

An ion source for generating an ion beam of primary ions is disclosed that includes a plasma chamber and magnets positioned therein for separating the primary ions of the plasma from secondary ions within the plasma. An electrode assembly extracts the primary ions through an extractor outlet port of the plasma chamber to form an ion beam, which preferentially is shaped as a ribbon beam. The primary ions are accelerated in the form of a ribbon beam toward the target workpiece for doping the device. The magnets are oriented in the chamber to produce a uniform current density of primary ions parallel to the elongated axis of the ribbon beam.

REFERENCE TO RELATED APPLICATIONS

The current application is a continuation-in-part of and incorporates byreference the commonly-owned, U.S. patent application Ser. No.08/601,983, for ION IMPLANTATION SYSTEM FOR FLAT PANEL DISPLAYS, filedon Feb. 16, 1996, abandoned.

This application is also related to and incorporates by reference thefollowing commonly assigned U.S. patent applications:

U.S. patent application Ser. No. 08/756,972, for METHODS AND APPARATUSFOR COOLING WORKPIECES (Attorney Docket No. ETE-002), filed the same dayherewith;

U.S. patent application Ser. No. 08/757,726, for CONTROL MECHANISMS FORDOSIMETRY CONTROL IN ION IMPLANTATION SYSTEMS (Attorney Docket No.ETE-003), filed the same day herewith;

U.S. patent application Ser. No. 08/756,656, for LARGE AREA UNIFORM IONBEAM FORMATION (Attorney Docket No. ETE-004), filed the same dayherewith;

U.S. patent application Ser. No. 08/753,514, for ION SOURCE SHIELDS(Attorney Docket No. ETE-008), filed the same day herewith;

U.S. patent application Ser. No. 08/756,133, for ION IMPLANTATION SYSTEMFOR IMPLANTING WORKPIECES (Attorney Docket No. ETE-010), filed the sameday herewith; and

U.S. patent application Ser. No. 08/756,372, for LOADLOCK ASSEMBLY FORION IMPLANTATION (Attorney Docket No. ETE-011), filed the same dayherewith.

BACKGROUND OF THE INVENTION

The present invention relates to ion implantation systems for large areaworkpieces, and more particularly, to ion implantation systems forforming large area ion beams for implanting flat panel displays.

Ion implantation has become a standard, commercially accepted techniquefor introducing conductivity-altering dopants into a workpiece, such asa semiconductor wafer or glass substrate, in a controlled and rapidmanner. Conventional ion implantation systems include an ion source thationizes a desired dopant element which is then accelerated to form anion beam of prescribed energy. This beam is directed at the surface ofthe workpiece. Typically, the energetic ions of the ion beam penetrateinto the bulk of the workpiece and are embedded into the crystallinelattice of the material to form a region of desired conductivity. Thision implantation process is typically performed in a high vacuum,gas-tight process chamber which encases a wafer handling assembly andthe ion source. This high vacuum environment prevents dispersion of theion beam by collisions with gas molecules and also minimizes the risk ofcontamination of the workpiece by airborne particulates.

The process chamber is typically coupled via a valve assembly with anautomated wafer handling and processing end station. The end station caninclude an intermediate chamber or pressure lock which can be pumpeddown from atmospheric pressure by a vacuum pumping system. This chamberpreferably communicates at one end with an end effector which transfersthe workpieces from one or more cassettes to the intermediate chamber.Once a workpiece has been loaded into the intermediate chamber by theend effector, the chamber is evacuated via the pump to a high vacuumcondition compatible with the process chamber. A valve at the downstreamend of the intermediate chamber then opens and the wafer handlingassembly mounted within the process chamber removes the workpiece fromthe chamber. After the workpiece is positioned in the chamber, it can beimplanted by the ion source.

Conventional ion sources consist of a chamber, usually formed fromgraphite, and having an input aperture for introducing a gas to beionized into a plasma. In general, the input gas that is ionized into aplasma consists of primary ions for implantation on the workpiece andsecondary ions that are a byproduct of the process. The extraction ofthe secondary ions into the ion beam is usually unwanted. One example ofsuch an input gas is phosphine, PH₃ which is utilized to produce P⁺ ionsfor doping the workpiece. The phosphine is diluted within the chamberwith hydrogen gas, and the mixture is ionized by bombardment with highenergy electrons. As a result of the ionization process, hydrogen ionsare produced which may be extracted from the chamber along with the P⁺ions. When the hydrogen ions impact the workpiece, they can produce anunwanted increase in temperature that may deform the substrate if thereis a sufficient current density of hydrogen ions. In order to reduce thenumber of hydrogen ions available for extraction at the electrode,magnets can be positioned within the chamber to force the hydrogen ionsaway from the extractor electrodes. These magnets are oriented, however,to produce a ribbon beam with a non-uniform current parallel to theelongated axis of the ribbon beam. The non-uniform current can thereforeform streaks of ion doping after implantation.

The typical elements in prior art ion implantation systems include anion source, electrodes, an analyzing magnet assembly, an opticalresolving element, and a wafer processing system. The electrodes extractand accelerate ions generated in the ion source to produce a beamdirected towards the analyzing magnet assembly. The analyzing magnetassembly sorts the ions in the ion beam according to theircharge-to-mass ratio, and the wafer processing system adjusts theposition of the workpiece along two axes relative to the ion beam path.

In particular, as each individual ion leaves the electrodes and entersthe analyzing magnets, its line of flight is bent into a path having aradius proportional to the square root of the mass of the ion. Aresolving slit in the analyzing magnet arrangement, in conjunction withthe optical resolving element, focus ions having a selectedcharge-to-mass ratio so that the ions are directed towards theworkpiece. Ions not having the selected charge-to-mass ratio are focusedeither to the left or to the right of the resolving slit and areselected out of the final ion beam striking the target workpiece.

In the earlier ion implantation machines, the ion source aperture wastypically a small hole approximating a point source. To achieve higherion beam currents, the size of the circular plasma aperture wasincreased, but it was soon discovered that there was a limit to theincrease in size of a circular aperture which would give an ion beam ofacceptable quality. Beam instability due to an unstable plasma meniscusresulted when both vertical and horizontal dimensions of the sourceaperture were simultaneously increased. However, it was found that, bylengthening the circular hole into a rectangular slit, higher beamcurrents could be obtained without beam instability. The rectangularslit was oriented perpendicular to the disbursive plane of the analyzingmagnet, tracking the parallel developments in obtaining higher currentsin isotope separators that utilized ion source exit slits with the sameorientation to the pole piece of the analyzing magnet.

To achieve higher and higher ion beam currents, the length of the ionexit aperture can be increased. In current practice, apertures capableof maintaining a stable plasma meniscus have been formed with a width inthe range of 1-3 mm. Prior art techniques that attempted furtherincreases in the size of the resolving have met with failure because ofthe beam blow-up phenomenon caused by use of the analyzing magnet inconjunction with larger slits. In addition, earlier systems that employanalyzing magnets in conjunction with larger slits are required toeither: (1) dramatically increases the size, expense, and powerrequirements of the analyzing magnet arrangement, or (2) space the ionsource further away from the analyzing magnets. Each of these approachescan lead to further undesirable side effects.

For example, as the ion beam source is moved further away, some of thegain in additional beam current from a longer ion exit slit is lost as agreater portion of the ion beam is neutralized in the longer line offlight region between the source and the analyzing magnet. To counteractthis, larger and more expensive vacuum pumps are required to avoidconversion of ions into neutralized species which cannot be analyzed.Thus, moving the ion source further from the analyzing magnet producescorresponding increases in overall size and complexity of the machinewhich directly translate into higher costs in manufacturing aninstallation.

Today's burgeoning semiconductor and implantation technology has foundwidespread acceptance in the marketplace. With this acceptance has comedemands for generating large quantities of implanted articles atcompetitive prices. A common goal of most modem implantation systems isto satisfy these demands by increasing the throughput of the system.Presently existing systems, however, are not well suited to meet thesemanufacturing and cost demands.

There exists a need for improved ion implantation systems. Inparticular, ion implantation systems that can limit secondary ions suchas during extraction of a ribbon beam would satisfy a long felt need inthe art,

SUMMARY OF THE INVENTION

The ion implantation system of the present invention achieves increasedthroughput by forming a ribbon beam from an electrode having a highaspect ratio. In particular, the ion implantation system has an ionsource including a chamber containing a plasma, and a plasma electrodeengaging an opening in wall of the ion chamber. The plasma electrode isfashioned as an elongated slot having a length at least fifty timeslonger than the width of the slot. The elongated plasma electrode shapesthe stream of ions exiting the ion chamber. Additionally, the ionimplantation system includes an electrode assembly downstream of theplasma electrode. The electrode assembly directs the stream of ionsexiting the ion chamber through the plasma electrode towards theworkpiece. The stream of ions directed towards the workpiece form aribbon beam.

In another aspect of the invention, the ion source for generating an ionbeam of primary ions in accordance with the invention includes a plasmachamber and magnets positioned therein for separating the primary ionsof the plasma from secondary ions within the plasma. An electrodeassembly extracts the primary ions through an extractor outlet port ofthe plasma chamber to form an ion beam, which preferentially is shapedas a ribbon beam. The primary ions are accelerated in the form of aribbon beam toward the target workpiece for doping the device. Themagnets are oriented in the chamber to produce a uniform current densityof primary ions parallel to the elongated axis of the ribbon beam. Sincethe current density is uniform, the streaking of the dopant on theworkpiece is eliminated.

The term "ribbon beam" as used herein includes an elongated ion beamhaving a length that extends along an axis of elongation and having awidth that extends along a second path transverse to the elongationaxis. Ribbon beams prove effective in implanting large area workpiecesbecause they can reduce the number of passes of the workpiece throughthe ion beam required to obtain a pre-selected dosage. For instance,prior art techniques required that the ion beam be scanned in twoorthogonal directions over the workpiece to completely cover theworkpiece. In comparison, when a ribbon beam has a length that exceedsat least one dimension of the workpiece, only one scan of the workpiecethrough the ribbon beam is required to completely cover the workpiece.The term high aspect ratio is used herein to describe ribbon beamshaving an aspect ratio of at least 50:1. Ribbon beams formed fromelectrodes having even higher aspect ratios of 100:1 may be producedusing the ion implantation system disclosed herein. These ribbon beamsprove useful in implanting larger workpieces, such as workpieces havingdimensions exceeding 550 mm by 650 mm.

According to another aspect of the invention, the ion implantationsystem uses a non-mass analyzed ribbon beam. It has been discovered thatthe ribbon beams capable of treating large area workpieces can be formedwithout beam blow-up by eliminating the analyzing magnets typically usedin forming ion beams.

Further features of the invention provide for large ranges in currentdensity and power in the ion beam. Techniques known in the art are notable to produce a ribbon beam as disclosed herein with the same rangesin current density and power. The invention accomplishes theseobjectives by providing for two or more slots in the plasma electrodeoriented substantially parallel to each other, with each slot havingaspect ratios that exceed 50:1. Accordingly, these slots form a splitextraction system that produces overlapping ribbon beams at the surfaceof the workpiece, i.e. a cumulative ribbon beam.

The cumulative ribbon beam produced by the split extraction system has acurrent density ranging from 0.02 to 100 microamps per squarecentimeter. In addition, the cumulative ribbon beam has a power thatranges from 1 to 100 kilowatts. The cumulative ribbon beam can befurther characterized by its size at the workpieces, that varies from 25centimeters to 1000 centimeters in length and that varies from 1millimeter to 250 millimeters in width. Preferably, the width of thecumulative ribbon beam varies from 10-15 centimeters at the surface ofthe workpiece.

It has been further discovered that the ribbon beams generated by thesystem disclosed herein are also highly collimated. The highlycollimated ion beams are particularly susceptible to disturbances in thepath of the ion beam. Disturbances, such as stray energy fields, curvesin the plasma meniscus, or particle contamination, causenon-uniformities to arise in the current density of the ribbon beam atthe surface of the workpiece. For instance, a particle contaminating theedge of an electrode might cause an ion stream to alter its originalpath from the ion source towards the workpiece. The altered path of theion stream can then cause an increase in current density at one point onthe workpiece and a decrease in current density at another point on theworkpiece. These slight changes when propagated through the implantationsystem give rise to non-uniformities. To overcome these problems theinvention can further include a diffusing system.

The diffusing system homogenizes the ion stream forming the ribbon beam.By homogenizing the ion stream the overall ribbon beam obtains increaseduniformity of current density along at least one axis of the ribbonbeam. For example, if the ion stream is highly collimated, then a slightdisturbance can affect the uniformity of the ribbon beam. In comparison,when the ion stream forming the ribbon beam is homogenized, then the ionstream intentionally includes ions moving along scattered, i.e.non-parallel, paths from the ion source towards the workpiece. Theseintentionally modified ion paths reduce the effects caused bydisturbances in the beam path, thereby reducing any non-uniformitiesarising at the workpiece.

The diffusing element can include an apertured plate having an array ofopenings and placed in the path of the stream of ions forming the ribbonbeam. The ion stream is homogenized as it passes through the array ofopenings in the apertured plate. Typically, the apertured plate ismounted within the opening in the ion chamber. Under thesecircumstances, the apertured plate is fashioned to be a plasma electrodethat has an array of openings for homogenizing the ion stream.

In accordance with another aspect of the invention, the apertured platecan include a plurality of arrays, each array being formed of a group ofopenings. A geometry associated with these arrays that increases theuniformity of the ion beam is governed by the expression, L<gNlΘ

where

L=the distance between openings in the apertured plate,

g=a constant in the range of 0 to 1,

N=the number of arrays in the apertured plate,

l=the distance between the apertured plate and the workpiece, and

Θ=the angular width of the stream of ions exiting at least one openingin the apertured plate.

Other features that enhance the homogenizing action of the aperturedplate have been discovered. According to a further aspect of theinvention, the openings included in the apertured plate are preferablyoriented in an array that extends along an axis substantially parallelto the length of the elongated slot. Further features of the inventionprovide for an array of openings having a plurality of elliptical oroval shaped openings, each ellipse or oval being oriented substantiallyparallel to each other. The elliptically shaped openings are separatedby a distance, D, in accord with the expression D=kL, where L is thelength of each elliptically shaped opening along the axis and where k isa constant in the range of 1/2 to 3/4. In other cases, the array ofopenings in the apertured plate may be circularly shaped and oriented ina plurality of staggered lines.

Additional aspects of the invention provide for alternative diffusingsystems capable of homogenizing the ions stream forming the ribbon beam.For instance, the diffusing system can include a magnet generating anoscillating magnetic field proximal to the path of the stream of ions;the diffusing system can include an electrode generating an oscillatingelectric field near the stream of ions; or the diffusing system caninclude a dithering magnet coupled with the ion chamber for causing theplasma to oscillate.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features and advantages of theinvention will be apparent from the following description and apparentfrom the accompanying drawings, in which like reference characters referto the same parts throughout the different views. The drawingsillustrate principles of the invention and, although not to scale, showrelative dimensions.

FIG. 1 is a perspective view of the ion implantation system according tothe present invention;

FIG. 2 is a top view of a portion of the ion implantation system of FIG.1;

FIG. 3 is a perspective view of the ion source shown in FIG. 1;

FIG. 3A is a side view of the ion source assembly with a cutaway so thatportions of the interior of the ion source may be viewed.

FIG. 3B is a section view of the ion source in accordance with thesection line 3B--3B of FIG. 3A.

FIG. 3C is a section view of the ion source in accordance with thesection line 3C--3C of FIG. 3A.

FIG. 4 is an overhead cross sectional view of a ribbon beam producedusing the ion source of FIG. 3;

FIG. 5 is a perspective view of a plasma electrode having two slots anda ribbon beam;

FIGS. 6A and 6B illustrate non-uniformities that can arise in the systemof FIG. 1;

FIG. 7 is an exploded view of a non-uniformity illustrated in FIG. 6A;

FIG. 8 shows the plasma electrode of FIG. 5 having three slots;

FIG. 9 is an exploded view of representative openings in the plasmaelectrode of FIG. 8;

FIG. 10 is an exploded view in perspective of representative openings inthe plasma electrode of FIG. 8;

FIG. 11 is an alternative embodiment of the plasma electrode of FIG. 8;

FIG. 12 is a schematic illustration of the ion source of FIG. 1 having adithering magnet;

FIG. 13 is a schematic illustration of the ion source of FIG. 1 having adiffusing electrode; and

FIG. 14 is a schematic illustration of the ion source of FIG. 1 having adiffusing magnet.

DESCRIPTION OF ILLUSTRATED EMBODIMENTS

The ion implantation system 10 of the present invention includes a pairof panel cassettes 26, an end effector 24, a loadlock assembly 12, ahousing 14 which defines a process chamber 16, and an ion source 18,which communicates with the process chamber 16 through beam aperture 20.An end effector 24 transfers panels P stacked in cassette 26 to theloadlock assembly 12.

The illustrated end effector 24 is coupled to convention drive andcontrol mechanisms which provide power to the end effector and whichcontrol the rate and sequence of panel movement. The panel cassettes 26are of conventional structure and provide a convenient storage facilityfor the panels.

The loadlock assembly 12 is also coupled to a linear bearing system anda linear drive system, which provide the desired vertical movement ofthe loadlock assembly 12 as well place and maintain hold the loadlockassembly 12 in sealing contact with the process chamber housing 14, asdescribed in further detail below. The linear drive system includes alead screw 22 and a motor assembly 23. The motor assembly 23 drives thelead screw 22 which in turn positions the loadlock assembly 12 at aselected vertical position, as indicated in dashed lines. The linearbearing system includes a pair of stationary linear bearings 28 mountedto the loadlock assembly which slide along circular shafts 29.

A translation assembly is preferably mounted within the process chamber16. The translation assembly includes a pickup arm 27 which is similarin design and function to the end effector 24. The pickup arm 27 handlesthe panel P during processing. When the pickup arm 27 initially removesthe panel P from the loadlock assembly 12, it is oriented in asubstantially horizontal position P1. The pickup arm then verticallyflips the panel, as denoted by arrow 13, into a substantially verticalposition P2. The translation assembly then moves the panel in a scanningdirection, from left to right in the illustrated embodiment, across thepath of an ion beam emerging from aperture 20, and which is generated bythe ion source 18.

With reference to FIGS. 1 and 2, the process chamber housing 14 includesa front housing portion 14A and a narrower elongated portion 14B. Thefront housing portion is sized to accommodate the removal of the panel Pfrom the loadlock assembly 12 in the horizontal position. The panel isthen moved from the original horizontal position P1 to the verticalposition P2 prior to movement along the scanning direction, indicated inFIG. 2 by arrow 32. The housing portion 14B has an axial dimension alongthe scanning direction that allows the panel to pass completely by theribbon beam generated by the ion source 18. The relatively narrow widthof the chamber portion 14B preferably allows the panel to move therealong only when disposed in the vertical position P2 to reduce the totalvolume of the process chamber 16. This reduction in chamber volumeallows faster evacuation of the process chamber. Reducing the timenecessary to evacuate the chamber serves to increase the totalthroughput of the implantation system 10.

The illustrated loadlock assembly 12 is preferably sealingly coupled tothe front chamber wall 34 of the chamber housing 14. The loadlockassembly 12 maintains a relatively pressure-tight and fluid-tight sealwith the chamber wall 34 during the vertical movement of the loadlockassembly 12. This sliding seal tight arrangement is described in furtherdetail below.

Referring again to FIG. 1, the ion source 18 forms a ribbon beam havinga long dimension that exceeds at least the smaller dimension of thepanel being processed. More specifically, the ion source generates aribbon beam whose length exceeds the panel's narrow dimension. The useof a ribbon beam in conjunction with the ion implantation system 10 ofthe present invention provides for several advantages, including (1) theability to process panel sizes of different dimensions with the samesystem; (2) achieving a uniform implant dosage by controlling the scanvelocity of the panel in response to the sampled the current of the ionbeam; (3) the size of the ion source can be reduced and is thus lessexpensive and easier to service; and (4) the ion source can continuouslybe operated. The continuous operation of the ion source increases theefficiency of the system 10 since it produces a more uniform implant byeliminating problems associated with turning the ion source off and on,as in prior approaches. These problems typically include beam currentdensity transients which occur upon start-up operation of the source.

FIG. 3 illustrates a perspective view of the ion source 18, including anion chamber 82 and an electrode assembly 91. The ion chamber 82 forms aclosed container for holding a plasma 84. The ion chamber has an opening60 at one end of the ion chamber. The opening allows ions to exit theion chamber 82 and form an ion stream 141. The ion stream 141 isdirected towards a workpiece 174 by the electrode assembly 91.

The opening 60, in the ion chamber, is covered by a plasma electrode 94.The plasma electrode is adapted for engaging the opening 60 and theplasma electrode has an aperture formed for shaping the stream of ionsexiting the ion chamber. In particular, the plasma electrode has a slotelongated along an axis 248. The elongated slot has a high aspect ratiofor shaping the stream of ions exiting the ion chamber 82 into a ribbonbeam.

The elongated slot has a high aspect ratio, that is the length of theslot along axis 248 greatly exceeds the width of the slot along atransverse axis. The high aspect ratio of the slot provides an ion beamcapable of treating a workpiece positioned below the ion beam with highcurrent densities and high energy levels. Generally, the length of theslot is at least fifty times the width of the slot. Preferably, thelength of the slot along axis 248 is at least one hundred times thewidth of the slot along a transverse axis. This very high aspect ratio,e.g. 100:1, forms a ribbon beam particularly applicable to theimplantation of large area workpieces.

FIG. 3A shows ion source 18 in further detail. Ion source 18 includes aplasma chamber 41, having a gas inlet 42 for introducing a source gas 43into the chamber 41. In chamber 41, source gas 43 is ionized into aplasma consisting of primary and secondary ions through interaction withhigh energy electrons produced by an electron source 44. The electronsource 44 may be a negatively biased tungsten filament which when heatedto a suitable temperature emits electrons, as is well known by those ofordinary skill in the art. Electrode assembly 91, consisting of a groupof four electrodes, extracts the primary ions through the extractoroutlet port 45 of the plasma chamber 41 to form ion beam 47, and directsthe ion beam 47 toward the target workpiece. The electrode assembly 91consists of the aperture electrode 94, the extractor electrode 96, thesuppression electrode 98, and the ground electrode 100.

Ion beam 47 implants ions on the target workpiece. In a preferredembodiment, the ion beam 47 is a ribbon beam with a uniform ion currentalong at least one axis of the beam. A more detailed description of theelectrode assembly 46 for extracting ions into a ribbon beam isdiscussed below.

An example of a source ion gas 43 is PH₃, which when diluted in theplasma chamber 41 with hydrogen gas generates a phosphine plasma withinthe chamber 41 having hydrogen ions, PH_(n) ⁺, and P⁺ ions. Theionization process within the plasma chamber 41 produces the primaryions that are extracted by the extractor electrodes 46 to form the ionbeam. As a consequence of the ionization process, secondary ions arealso produced which if incorporated in the extracted beam would createundesirable thermal activity during the implantation process and defectson the surface of the workpiece material. Using the previous example ofPH₃ as the source ion gas, the P⁺ and PH_(n) ⁺ ions are the primary ionsource constituting the ion beam. As a byproduct of the ionizationprocess, hydrogen ions are also generated as the secondary ions. Duringthe implantation process, secondary ions in the beam may create defectsin the workpiece structure due to increased thermal activity on thesurface of the workpiece.

In order to reduce the unwanted thermal activity associated with thepresence of hydrogen ions in the beam during the implantation process, amagnetic assembly is positioned within the plasma chamber 41 to separatethe secondary ions from the primary ions which form the ion beam. Asshown in FIG. 3B, the magnetic assembly consists of a multiplicity ofpermanent magnets 48 which are oriented in the chamber 41 so that themagnetic field generated by the magnets 48 is perpendicular to thelongitudinal axis of the chamber as shown by line A--A and at the sametime is parallel to the elongated axis, shown as the dashed line B--B inthe drawings, of the generated ribbon beam 47. This orientation of themagnets 48 promotes a uniform current density parallel to the elongatedaxis B--B of the ribbon beam 47, and thus avoids striping the workpiecewith the ion beam.

Referring still to FIG. 3B, magnets 48 are arranged such that the northpole of one magnet 48 is positioned adjacent to the south pole ofanother magnet 48. Magnets 48 generate a magnetic field that forces themore energetic electrons to remain in region 52, and thus the secondaryions of the plasma avoid the vicinity of the extractor outlet port 45.The possibility is reduced that these ions will be extracted into theribbon beam 47 by the extractor electrodes 46.

The orientation of the permanent magnets 48 causes the primary andsecondary ion mixtures to split and form separate plasmas in the chamber41. Secondary ions are forced away from the extractor electrodes 46 intoplasma pool 52. Due to the distance of the secondary ions from theextractor electrodes 46, very few are extracted into ribbon beam 47. Theribbon beam consists of 70% primary ions and more preferably over 90%primary ions. Primary ions accumulate into a separate plasma pool 54.Due to the vicinity of plasma pool 54 to the extractor electrodes 46,primary ions are readily available to be extracted in to ion ribbon beam47.

As shown in FIG. 3C, permanent magnets 49 surround the exterior of thechamber 41. Magnets 49 confine the plasma in a quiesent state with auniform plasma density profile. Magnets 49 surround the plasma chamber41 in an alternating north-pole south-pole orientation to passivate theinterior chamber surface by reducing the ion interaction s with theinterior walls.

In ion implantation system 10 the length of the slot is determined as afunction of the width of the workpiece. The length of the elongated slotin the plasma electrode 94 must be longer than the width of theworkpiece to eliminate the need for a two directional workpiece scanningsystem. Furthermore, the aspect ratio between the width and the lengthof the plasma electrode preferably exceeds 50:1 to achieve an acceptablelevel of control over the ion flow from the ion source.

For instance, if the slot in the plasma aperture is elongated and theslot remains wide then the ions flow through the slot in an uncontrolledmanner. In comparison, when the slot is long and the slot remainsnarrow, the ions flow through the slot in a controlled manner. Slotshaving a high aspect ratio reduce gas flow from the source and allow thesource to operated at high plasma densities for a given dose. Thisallows the source to operated over a wider range of current densities.Accordingly slots having aspect ratios exceeding 50:1, and preferablyexceeding 100:1, are most suited for implanting large areas workpieces.

The electrode assembly 91, placed downstream of the plasma electrode 94,includes an extraction electrode 96, a suppression electrode 98 and aground electrode 100. The electrode assembly directs and accelerates theions in the ion stream 141 towards the workpiece. Typically, a set ofvariable voltage supplies (not shown) regulate the voltages between theelectrodes 94, 96, 98, and 100. The set of variable voltage supplies areadjusted so that the plasma electrode, the extraction electrode, and thesuppression electrode are at a selected voltage relative to the groundelectrode. For instance, the suppression electrode 98 is adjusted to beminus three kilovolts, the extraction electrode is adjusted to be at 90kilovolts, and the plasma electrode is typically adjusted to be at 95kilovolts. The voltages can be adjusted to make adjustments to theenergy and focus of the ion stream 141.

FIG. 4 illustrates an overhead cross sectional view of the ribbon beam141 at the surface of the workpiece 174. The workpiece is supported by aworkpiece platform. The workpiece platform, as shown, includes twosubstantially parallel rails 244A and 244B and a table 242 mounted tothe rails 244A and 244B. The table 242 can be mounted to the rails usingslidable mounts known in the art, such as a linear bearing or an airbearing. The table 242 is mounted in a fashion that allows the table tomove along axis 246 and that allows a motor (not shown) to drive thetable 242 along the direction of axis 246.

Preferably, the ion source 18 generates a ribbon beam 141 having a crosssectional area as shown in FIG. 4. The ribbon beam is an ion beamelongated along one axis. The axis of elongation of the ribbon beam islabeled as axis 248 and is shown to be substantially perpendicular tothe direction of movement of the workpiece 174 along axis 246.

The length of the ribbon beam 141 along axis 248 exceeds at least thenarrower dimension of the workpiece 174. This distinction advantageouslyallows a workpiece traveling along axis 246 and approximately centeredin the ribbon beam to be completely exposed to the ion stream during onepass through the ribbon beam. Thus, the ribbon beam eliminates the needfor a complex two directional scanning system that moves the workpiecealong a scanning direction and an orthogonal direction.

FIG. 5 is a perspective view of a plasma electrode 94' that is adaptedfor engaging the opening in the ion chamber 82. The plasma electrode 94'has a first slot 60 and a second slot 62, both of which are elongatedalong an axis substantially parallel to axis 248. A stream of ions fromthe ion chamber 82 passes through both the first slot 60 and the secondslot 62 to treat the workpiece 174.

A first stream of ions passing through the first slot 60 forms an ionbeam 61 in the shape of a ribbon beam. A second stream of ions passingthe second slot 62 forms a second ion beam 63 in the shape of a ribbonbeam. As a result of the elongated shape of slots 60 and 62, the ionbeams are formed into the shape of ribbon beams 61 and 63 extendingalong axis 248.

Preferably, slots 60 and 62 are separated by a distance that causes theribbon beams 61 and 63 to minimally overlap at the surface of theworkpiece 174. The first and second ribbon beams 61, 63 form acumulative ribbon beam 65. The cumulative ribbon beam 65 can be formedof two or more ribbon beams.

The cumulative ribbon beam 65 has a greater range in current density andenergy than a single individual ribbon beam typically achieves. Forexample, the cumulative ribbon beam 65 can have a current density in therange of 0.02 to 100 μAmps/cm². The energy of the ions in the cumulativeribbon beam can be varied in the range of 1-100 kiloelectron volts(keV).

Moreover, the use of a plurality of slots 60, 62 in the plasma electrode94' provides for greater variability in the size of the ribbon beam 65at the surface of the workpiece 174. For instance, a single ribbon beammay be limited in the width of the ribbon beam achievable along axis246. In comparison, a cumulative ribbon beam formed of multiple ribbonbeams aligned parallel to axis 248 can theoretically achieve acumulative ribbon beam of infinite width extending along axis 246. Theincreased width of the ribbon beam advantageously provides for anincreased rate of doping per unit time.

The cumulative ribbon beam 65 can have a length that varies from 25 to1000 cm. at the surface of the workpiece. The cumulative ribbon beam canalso have a width that varies from 1 mm. to 250 mm. at the surface ofthe workpiece. Preferably, the width of the cumulative ribbon beam alongaxis 246 varies between 10 and 30 cm. at the surface of the workpiece.The preferred width of the ribbon beam, between 10 and 30 cm., providesfor an ion stream that efficiently and effectively doses the workpiecewithout causing the workpiece to overheat or be subjected tonon-uniformities in current density.

FIGS. 6A and 6B illustrate non-uniformities that can arise in currentdensity at the workpiece 174 using an ion implantation system. Inparticular, FIG. 6A illustrates a representative graph of currentdensity along a path parallel to axis 248 at the surface of theworkpiece, and FIG. 6B illustrates a representative graph of currentdensity along a path parallel to axis 246 at the surface of theworkpiece. A representative non-uniformity is labeled as item number 69in FIG. 6A. In a perfect ion implantation system, the current density atthe workpiece constantly equals the nominal value J₁.

In ion implantation system 10, maintaining a near constant currentdensity along a path parallel to axis 248 proves to be more criticalthan maintaining a near constant current density along a path parallelto axis 246. If the current density varies as shown in FIG. 6B, e.g.along axis 246, then various points on the workpiece along axis 174 atany point in time will receive a different ion dosage. However, as theworkpiece progresses along axis 246, each point on the workpiece willpass through those areas in the ion beam having high current density andthose areas having low current density. Accordingly, each point on theworkpiece receives the same total dosage after completely passingthrough the ion beam. If the current density varies as shown in FIG. 6A,e.g. along axis 248, then various points on the workpiece along axis 248at any point in time receive a different ion dosage, either high, low,or nominal. As the workpieces progresses along axis 246, each pointcontinues to receive the same dosage, either high, low, or nominal.Accordingly, stripes of varying dosage that run substantially parallelto axis 246 form on the workpiece, each of the stripes corresponding toan area of high, low, or nominal dosage.

The invention provides systems for homogenizing the ion stream to removethese non-uniformities in current density, as discussed hereinafter. Thesystems for controlling uniformity of the ion beam can be applied toreduce or remove the non-uniformities as shown in FIGS. 6A or 6B. Thehomogenizing systems, however, are used primarily to remove thenon-uniformities shown in FIG. 6A for the reasons discussed above.

FIG. 7 shows an exploded view of the representative non-uniformity 69and schematically illustrates one possible cause of the non-uniformity69. In particular, a plasma electrode 94 is positioned over the openingin an ion chamber (not shown). The plasma electrode has a slot 60 thatcauses the formation of a plasma meniscus 70. The meniscus has a convexbump 71 positioned in approximately the center of the meniscus.Illustrative ion paths 72, 74, 76, 78, 80, 82 are shown projecting fromthe plasma meniscus 70 towards the workpiece 174.

The bottom half of FIG. 7 shows a graph of current density versusposition along the workpiece 174. On the graph the representativenon-uniformity 69 is shown. In addition, the relationship between thenon-uniformity 69 and the ion beam paths 72, 74, 76, 78, 80, and 82 isshown with dashed lines extending from the workpiece 174 to the graph.For instance, a relative minimum in current density 86 is associatedwith the convex hump 71 in the plasma meniscus 70. A first relative highin the current density 84 is associated with ion paths 74 and 74, and asecond relative high in current density 88 is associated with ion paths78 and 80. Nominal values in current density 85 and 87 are associatedwith ion paths 72 and 82, respectively.

FIG. 7 illustrates how a disturbance in the path of the ion beam canadversely effect the uniformity of the current density at the workpiece174. For example, the hump 71 in the plasma meniscus 70 can cause awiggle in the current density at the workpiece consisting of a valley 86and two peaks 84, 88. It is understood that the ion stream exiting theion source through the opening 60 in the aperture 94 is a highlycollimated ion stream. As a result, changes in the path of a relativelysmall number of ions can dramatically effect the uniformity of thecurrent density at the workpiece.

For instance, ion path 72, 74, 80 and 82 are all substantially parallelion paths as they exit the opening 60 in aperture in electrode 94. Thesepaths create a substantially uniform current density at the workpiece174. In comparison, the hump 71 causes ion paths 76 and 78 to vary fromthe substantially parallel ion paths of the other ion beams. The ionbeams 76 and 78, however, straighten out as they proceed towards theworkpiece 174. As a result, ion beam 76 then overlaps with the path ofion beam 74, and ion beam 78 overlaps with the path of ion beam 80. Theoverlapping of paths creates the relative high in current density atpeaks 84, 88 and the relative low in current density at valley 86. Thevariations in the paths of beam 76 and 78 caused by the plasma meniscushump 71 results in a non uniform current density.

This example identifies how minor local variations in the plasma densitycan cause non-uniformities in the current density at the workpiece 174.Other minor defects in the ion implantation system can have similareffects on the current density. For instance, contaminant or particlebuild-up on electrodes both creates minor variations in the electricfield generated by these electrodes and causes minor mechanicalprotrusions into the ion stream from the electrodes. These slightdisturbances, however, can be sufficient to create non-uniformitiessimilar to that illustrated in FIG. 7.

Non-uniformities in current density of the type described above canprove problematic in the treatment of a workpiece 174. In particular,variations in the current density along axis 248 can create stripes ofvarying dosage in the workpiece 174. A relative high in current densityalong axis 248 will produce a high dosage stripe in the workpiece 174along axis 246. A relative low in current density along axis 248produces a low dosage stripe in the workpiece along axis 246.

FIG. 8 illustrates a plasma electrode 95 having a diffusing system forreducing the non-uniformities in the ion stream. The inventorsdiscovered a number of exemplary diffusing systems that reducenon-uniformities in current density introduced by the disturbances suchas those discussed above in FIG. 7. Each of the diffusing systemsimproves the uniformity of current density along at least one axis ofthe workpiece by homogenizing the ion stream. Preferably, the diffusingsystems enable the ion implantation system to obtain a current densitythat varies by less than 0.5% along at least one axis of the workpiece.

FIG. 8 illustrates an apertured plate 95 that can be used as a diffusingelement for homogenizing the ion stream. The apertured plate 95 can besituated in the stream of ions anywhere between the ion source 18 andthe workpiece 174. Preferably, the apertured plate is mounted within theopening 60 of the ion chamber 82. For instance, the apertured plate 95can be shaped and adapted for engaging the opening 60 of the ion chamber82, similar to technique whereby the plasma electrode 94 is adapted forengaging the opening 60 in the ion chamber 82.

The apertured plate 95 has a first array of openings 110, a second arrayof openings 112, and a third array of openings 114. The array ofopenings 110, 112, and 114 all extend along an axis substantiallyparallel to axis 124. Other aspects of the invention can provide for anapertured plate 95 having only a single array of openings extendingalong axis 124, such as array 110. The aperture plate 95 is placed inthe ion stream such that the ion stream passes through the openings inthe apertured plate, thereby homogenizing the ion stream to provide fora substantially uniform current density along at least one axis.

In the preferred embodiment where the apertured plate is mounted to theopening 60 of the ion chamber 82, the apertured plate 95 acts as aplasma electrode. The apertured plate provides a first elongated slot111, a second elongated slot 113, and a third elongated slot 115. Slot111 includes the first array of openings 110, slot 113 includes thesecond array of openings 112, and slot 115 includes the third array ofopenings 114. A representative group of openings is identified as itemnumber 121. Each of the slots 111, 113, 115 separately allows a streamof ions to pass through to form a separate ribbon beam. Each of theribbon beams produced by slots 111, 113, 115 overlap at the surface ofthe workpiece 174 to form a cumulative ribbon beam. In addition, due tothe array of openings in each of the slots, each of the ribbon beamsproduced has a substantially uniform current density along at least oneaxis.

Further in accordance with this aspect of the invention, the electrodeassembly 91 of FIG. 3 includes electrodes 96, 98, and 100 that differfrom the plasma electrode 95. In particular, the disclosed plasmaelectrode includes one or more slots, each having an array of openings,while the electrodes 96, 98, and 100 do not include arrays of openings.Rather, the electrodes 96, 98, and 100 are formed as elongated slotswithout an apertured plate. The electrodes 96, 98, and 100 are simplyformed of elongated slots having a high aspect ratio.

FIG. 9 shows an exploded view of the representative group of openings121 in FIG. 8. The group of openings includes a first opening 116, asecond opening 117, and a third opening 119. The openings 116, 117, and119 form an array that extends along the axis 124. Each of the openingsin the array are in the shape of an ellipse. In addition, each of theelliptically shaped openings are oriented substantially parallel to theother openings in the array. The openings 121 prove particularly helpfulin homogenizing the current density the ion beam along an axissubstantially parallel to the axis 124 at the surface of the workpiece.This diffusing system achieves a current density that varies by lessthan 0.5% along axis 124 at the surface of the workpiece.

In accordance with one aspect of the invention, the array of oval orelliptical openings extending along axis 124 are governed by aparticular mathematical expression such that the array reducesnon-uniformities in the ion beam. Those openings that conform to thefollowing mathematical expression reduce non-uniformities because thebeamlets produced by each of the openings overlap at the surface of theworkpiece. In particular, the openings are governed by the equation:

    D=kL                                                       (Equation A)

where "k" is a constant in the range of 1/2 to 3/4.

FIG. 9 shows the graphical representations for "D" and "L" in EquationA, relative to the array of openings. In particular, "D" is the distancebetween the foci of adjacent elliptically shaped openings in the array112; and "L" is the length of each elliptically shaped opening along theaxis of elongation of array 112. That is, "L" equals the distance of theopening 117 along axis 124 of the array 112.

FIG. 10 shows an exploded view in perspective of the representativegroup of openings 121 in FIGS. 8 and 9. The openings 121 are located onthe apertured plate 95. The representative openings 121 in the aperturedplate 95 form an array that extends along axis 124, and the array shownincludes the opening 116, the opening 117, and the opening 119.

FIG. 10 also illustrates, in perspective, the workpiece 174 positionedbelow the apertured plate 95 and separated from the plasma electrode bya distance "l". The ion beam projecting through opening 117 has anangular width "θ", and the ion beam projecting through opening 117 formsa beam having a width "d" at the surface of the workpiece 174.

The inventors have also discovered that an apertured plate having amultiple number of slots, such as slot 111, 113, and 115 of FIG. 8, canhave varying degrees of uniformity depending upon the geometry of theslots in the apertured plate. Particularly, an apertured plate havingopenings that abide by the following equation will produce a ribbon beamhaving an improved level of uniformity at the surface of the workpiece174. Those openings that conform to the following mathematicalexpression reduce non-uniformities because the beamlets produced by eachof the openings overlap at the surface of the workpiece.

    L<gNlθ                                               (Equation B)

With reference to Equation B, "L" is the distance between the openingsalong axis 124, "g" is a constant in the range of 0 to 1, "l" is thedistance between the apertured plate 95 and the workpiece 174, and "N"is the number of slots in the aperture. For instance, the number ofslots "N" shown in FIG. 8 in aperture 95 is three. With furtherreference to Equation B, "θ" is the angular width of the beamlet exitingan opening, and can be approximated as "d" divided by "l". Typically theangular width of the beamlet is measured along the direction of axis124. Equation B, in comparison to Equation A, is more generic. EquationB applies to all openings, regardless of shape, while Equation A is moresuited for those instance where elliptical or oval openings areutilized.

FIG. 11 illustrates an apertured plate 95' having a first slot 111' asecond slot 113' and a third slot 115'. The first slot 111' has a firstarray of openings 110', the second slot 113 has an array of openings112', and the third slot 115' has an array of openings 114'. Each arrayof openings 110', 112', 114', extends along an axis substantiallyparallel to axis 124. The apertured plate 95' acts as a diffusingelement for homogenizing an ion stream when placed in the path of theion stream. When the aperture plate is positioned in the ion stream, theion stream passes through the openings in the apertured plate such thatthe ion stream is homogenized and forms an ion beam having substantiallyuniform current density along at least one axis. In the preferredembodiment the apertured plate 95' is mounted to the opening 60 of theion chamber 82, and the apertured plate 95 acts as a plasma electrode.

Each of the slots 111', 113', and 115', separately produce an elongatedribbon beam when placed in the path of the ion stream. Each of theribbon beams overlap and form a cumulative ribbon beam at the surface ofthe workpiece 174. In addition, due to the array of openings in each ofthe slots, each of the ribbon beams produced has a substantially uniformcurrent density along at least one axis.

FIG. 11 also shows that the array of openings 110', 112', 114' areformed of a first row 118 of circular openings and a second row 120 ofcircular openings. Both rows 118 and 120 extend along a pathsubstantially parallel to axis 124. In addition, each individual openingin row 120 is oriented to the right and below an individual opening inrow 118. This orientation of the individual openings in rows 118 and 120creates an overall structure having an array of staggered openings thatextends along a path substantially parallel to axis 124.

FIG. 12 illustrates another feature of the diffusing element forhomogenizing the ion stream. In particular, FIG. 12 shows a ditheringmagnet system that homogenizes the ion stream to produce a ribbon beamhaving a substantially uniform current density. The ion chamber 82contains a plasma 84. The ion chamber 82 also includes the opening 60through which the ion stream exits and forms the ion beam 141. A plasmaelectrode 94 is situated over the opening 60 to aid in controlling theions exiting the ion chamber 82. The electrode assembly 91 has anextraction electrode 96, a suppression electrode 98 and a groundelectrode 100. The electrode assembly directs the ion beam 141 towardsthe workpiece 174.

FIG. 12 also illustrates a first dithering magnet 130A, a seconddithering magnet 130B, a third dithering magnet 130C, and a fourthdithering magnet 130D mounted to the ion chamber 82. The ditheringmagnets are typically formed of coils wrapped around a metallic core,thus forming an electromagnet having a first end and a second end. Thedithering magnets are electrically coupled to an alternating currentsource 134 in the manner shown in FIG. 12. In particular, the output ofthe current source is connected to the first end of magnet 130B, and thesecond end of magnet 130B is connected to the second end of magnet 130A.The first end of magnet 130A is connected to the first end of magnet130C, and the second end of magnet 130C is connected to the second endof magnet 130D. The first end of magnet 130D is connected the return ofthe current source 134.

The current source 134 is variable and causes the dithering magnets130A, 130B, 130C, and 130D to oscillate. The oscillation of ditheringmagnets 130 and 132 creates an oscillating magnetic in the plasma 84.The oscillating magnetic field in the plasma causes homogenization ofthe ions in the ion stream 141. This homogenization lessens thesusceptibility of the ion stream to disturbances capable of formingnon-uniformities in the current density at the workpiece 174.

FIG. 13 illustrates another feature of the diffusing element forhomogenizing the ion stream. In particular, FIG. 13 shows a ditheringelectrode system that homogenizes the ion stream to produce a ribbonbeam having a substantially uniform current density. The ion chamber 82contains a plasma 84. The ion chamber 82 also includes the opening 60through which the ion stream exits and forms the ion beam 141. A plasmaelectrode 94 is situated over the opening 60 to aid in controlling theions exiting the ion chamber 82. The electrode assembly 91 has anextraction electrode 96, a suppression electrode 98 and a groundelectrode 100. The electrode assembly directs the ion beam 141 towardsthe workpiece 174.

The dithering electrode system includes an alternating current source134 connected to a dithering electrode 135. The dithering electrode ismounted to the ion source 82. Typically the dithering electrode 135 issituated proximal to the plasma 84. The dithering electrode coupled inconjunction with the alternating current source 134 forms an oscillatingelectric field proximal to the plasma 84. The oscillating electric fieldcauses homogenization of the ions in the ion stream 141. Thishomogenization lessens the susceptibility of the ion stream todisturbances capable of forming non-uniformities in the current densityat the workpiece 174.

Preferably, the frequency of oscillation of the dithering electrode isgoverned by the following expression: ##EQU1## With further reference toEquation C, "F" is the frequency of the oscillating electric fieldgenerated by the dithering electrode, "n" is a constant in the range of10 to 100; "V_(scan) " is the speed of movement of a workpiece throughthe ribbon beam 141, and "W" is the width of the ribbon beam at theworkpiece.

FIG. 14 illustrates another feature of the diffusing element forhomogenizing the ion stream. In particular, FIG. 14 shows a diffusingmagnet system that homogenizes the ion stream to produce a ribbon beamhaving a substantially uniform current density. The ion chamber 82contains a plasma 84. The ion chamber 82 also includes the opening 60through which the ion stream exits and forms the ion beam 141. A plasmaelectrode 94 is situated over the opening 60 to aid in controlling theions exiting the ion chamber 82. The electrode assembly 91 has anextraction electrode 96, a suppression electrode 98 and a groundelectrode 100. The electrode assembly directs the ion beam 141 towardsthe workpiece 174.

The diffusing magnet system includes a first diffusing magnet 130A, asecond diffusing magnet 130B, a third diffusing magnet 130C, and afourth diffusing magnet 130D. The diffusing magnets are typically formedof coils wrapped around a metallic core, thus forming an electromagnethaving a first end and a second end. The diffusing magnets areelectrically coupled to an alternating current source 134 in the mannershown in FIG. 12. In particular, the output of the current source isconnected to the first end of magnet 136A, and the second end of magnet136A is connected to the second end of magnet 136B. The first end ofmagnet 136B is connected to the first end of magnet 136C, and the secondend of magnet 136C is connected to the second end of magnet 136D. Thefirst end of magnet 136D is connected the return of the current source134.

The alternating current source 134 is variable and causes the diffusingmagnets 136A, 136B, 136C, and 136D to oscillate. The diffusing magnet136 in conjunction with the variable current source forms an oscillatingmagnetic field proximal to the stream of ions forming the ion beam. Theoscillating magnetic field causes homogenization of the ions in the ionstream 141. This homogenization lessens the susceptibility of the ionstream to disturbances capable of forming non-uniformities in thecurrent density at the workpiece 174.

Preferably, the frequency of oscillation of the diffusing magnets isgoverned by the following expression: ##EQU2## With further reference toEquation D, "F" is the frequency of the oscillating magnetic fieldgenerated by the diffusing magnets, "n" is a constant in the range of 10to 100; "V_(scan) " is the speed of movement of a workpiece through theribbon beam 141, and "W" is the width of the ribbon beam at theworkpiece.

The B-field, designated by the letter "B" and three parallel arrows isillustrated in FIG. 14. The B-field has a height "d_(B) ", as also shownin FIG. 14. Preferably, the B-field generated by the diffusing magnetsdecreases uniformities in the ribbon beam along the length of the ribbonbeam. For instance, the B-field reduces non-uniformities in currentdensity along a path substantially parallel to axis 248 at the surfaceof the workpiece.

Further in accordance with this preferred embodiment of the diffusingmagnet system, the B-field is directed through the ion beam 141 along apath substantially parallel to axis 246. A B-field directed along thispath causes the ions in the ion beam 141 to dither along the directionof axis 248. This dithering increases the uniformity of the ion beamalong the length of the ribbon beam, i.e. axis 248.

The strength of the B-field required to homogenize the ion beam variesas a function of the extraction voltage applied to extraction electrode96, the ion mass, and the angular width θ of the ion beam 141, as shownin FIG. 10. Accordingly, the strength of the B-field is in the range of500-3000 Gauss-centimeter per d_(B), and is preferably 2800Gauss-centimeter per d_(B).

It will thus be seen that the invention efficiently attains the objectsset forth above, among those made apparent from the precedingdescription. Since certain changes may be made in the aboveconstructions without departing from the scope of the invention, it isintended that all matter contained in the above description or shown inthe accompanying drawings be interpreted as illustrative and not in alimiting sense.

It is also to be understood that the following claims are to cover allgeneric and specific features of the invention described herein, and allstatements of the scope of the invention which, as a matter of language,might be said to fall there between.

Having described the invention, what is claimed as new and desired to besecured by Letters Patent is:
 1. Apparatus for controlling theextraction of an ion beam from a plasma having a mixture of primary ionsand secondary ions, said apparatus comprising:a chamber for enclosingsaid plasma, said chamber having an outlet port, a magnetic fieldgenerating means for separating said primary ions from said secondaryions within said chamber, and an extractor coupled to said outlet portfor forming a ribbon shaped ion beam elongated along a first axis,wherein said extractor cooperates with said magnetic field generatingmeans to extract a ribbon shaped ion beam of the primary ions the ribbonshaped ion beam having reduced non-uniformities.
 2. The apparatus ofclaim 1 wherein said magnetic field generating means generates amagnetic field parallel to said first axis.
 3. The apparatus of claim 2wherein said magnetic field generating means comprises a plurality ofpermanent magnets arranged such that the north pole of one magnet isadjacent to the south pole of a second magnet.
 4. The apparatus of claim1 wherein said magnetic field generating means is enclosed by saidchamber.
 5. The apparatus of claim 1 wherein said secondary ions arehydrogen ions.
 6. The apparatus of claim 1 wherein said primary ions areP⁺ ions.
 7. The apparatus of claim 1 wherein said chamber is encompassedby a plurality of magnets for reducing the plasma interaction with thewall of the chamber.
 8. An apparatus for controlling the extraction ofan ion beam from a plasma having a mixture of primary ions and secondaryions, the apparatus comprising:a chamber for enclosing the plasma, thechamber having an outlet port, an extractor coupled to the outlet portfor forming a ribbon beam of ions extending along an axis of elongation,and a magnetic field generator forming a magnetic field extendingsubstantially parallel to the axis of elongation, the magnetic fieldacting to separate the primary ions from the secondary ions within thechamber, wherein the extractor cooperates with the magnetic fieldgenerator to extract a ribbon beam of primary ions.
 9. The apparatus ofclaim 8 wherein the magnetic field generator comprises a plurality ofpermanent magnets arranged such that the north pole of one magnet isadjacent to the south pole of a second magnet.
 10. The apparatus ofclaim 8 wherein the magnetic field generator is enclosed by the chamber.